The present invention generally relates to electronic diplexers, and more particularly to tunable diplexers.
Commercially available radio frequency (RF) diplexers include two fixed bandpass filters sharing a common port (antenna port) through a circulator or a T-junction. Signals applied to the antenna port are coupled to a receiver port through the receive bandpass filter, and signals applied to a transmitter port will reach the antenna port through a transmit filter. The receive port and transmitter port are isolated from each other due to the presence of the filters and the circulator, or T-junction. In another configuration, the receive signals reaching the antenna will be divided into two sub-bands according to the band pass frequencies of the filters. In the opposite direction, two signals reaching the non-common ports of the filters will be combined and output at the common port. Also in this configuration the two filters are isolated with respect to each other.
Fixed diplexers are commonly used in point-to-point and point-to-multipoint radios where two-way communication enables voice, video and data traffic within the RF frequency range. Fixed diplexers need to be wide band so that their count does not exceed reasonable numbers to cover the desired frequency plan.
It would be desirable to have a tunable diplexer that would could be used to replace fixed diplexers in receivers. A single tunable diplexer solution would enable radio manufacturers to replace several fixed diplexers covering adjacent frequencies. This versatility can provide front end RF tunability in real time applications and decrease deployment and maintenance costs through software controls and reduced component count.
Diplexers constructed in accordance with this invention include a first tunable bandpass filter connected to a first port, a second tunable bandpass filter connected to a second port, and a coupling element for coupling the first bandpass filter and the second bandpass filter to a third port. Each of the tunable bandpass filters includes at least one tunable capacitor, wherein a control signal applied to the tunable capacitor controls the transmission characteristic of the filter. The tunable capacitor can be a tunable dielectric varactor or a microelectromechanical variable capacitor. The coupling element can include one of: a circulator, a T-junction, and an orthomode transducer. Each of the first and second filters can comprise a fin line filter including a plurality of tunable dielectric capacitors mounted within a waveguide for controlling the filter transmission characteristics. Fixed bandpass filters can be inserted between each of the tunable bandpass filters and the coupling element.